The present invention generally relates to the field of radio-frequency receivers, and more particularly to a method for increasing signal acquisition in global positioning system receivers.
Conventional global positioning system (GPS) receivers integrate the baseband quadrature components, implement a power detection function, and then follow this with post detection summation. If the frequency error is large, the signal will not be acquired because of the effective bandwidth of the coherent integration process. This requires multiple searches. A solution to the problem is to shorten the sampling interval and implement a banked filter process such as a Fast Fourier transform (FFT). The multiple outputs from the FFT can then be detected and summed in post detection filters. However, the Fast Fourier transform has several disadvantages. First, if the signal frequency falls between FFT frequency bins, it will be significantly attenuated. Second, if the signal frequency is near the outer edges of the FFT response, it will be attenuated by the integration process prior to sampling. Third, the FFT requires multiplication if more than four points are used. Fourth, utilization of an FFT typically produces frequency bins with frequencies up to plus and minus one-half the interval between samples, thereby causing the frequency bins on the extremes of the pattern to be unusable because of the attenuation caused by integration over the segment period.